This invention relates to a process and apparatus for the purification of industrial waste gases arising out of, for example, sintering processes, incineration of refuse, fossil substances and the like wherein such gases contain, in addition to fly ash, gaseous constituents which pollute the atmosphere. More particularly, the gaseous constituents of the waste gases include sulfur dioxide and sulfur trioxide as well as hydrogen fluoride, hydrogen chloride and nitrogen oxides.
Heretofore, wet processes were employed for the purification of such forms of industrial waste gases. However, the known forms of wet processes have the disadvantage that problems of environmental pollution are transformed from a gaseous state to a water state. The environmental problems which occur are particularly related to disposal problems which are difficult to solve, i.e., the disposal of thixotropic sludges or the disposal of salt solutions which produce the salting of water.
Under these conditions, it is obvious to employ only dry purification processes which do not give rise to water polluting problems as well as other dumping problems. An attempt has already been made in the art to use dry processes for the removal of sulfur dioxide and sulfur trioxide by the addition of ammonia. In British Patent No. 826,221, there is disclosed a process for the removal of sulfur dioxide from waste gases by means of gaseous ammonia wherein the reaction process is carried out above the gas dew point. A dry salt mixture is obtained which contains sulfite, hydrogen sulfite, sulfate and hydrogen sulfate. The dry salts are separated by means of electrostatic precipitators or cyclones. After the resulting salt mixture is dissolved, the solids are removed and the mixture is treated with sulfuric acid to produce sulfate with the sulfur dioxide being split off. The object of this process is the recovery of ammonium sulfate.
The reaction of sulfur dioxide with ammonia in damp air at room temperature on a laboratory scale is reported in pages 67-72 of the journal entitled "Ind. Eng. Chem. Fundam.", Vol. 14, No. 1 (1975). It is shown that the oxidation of sulfur dioxide to form sulfur in sulfate form under the laboratory scale conditions is influenced apart from oxygen, solely by the water vapor content of the gas mixture. The author of this article confines himself to room temperature conditions and points out that his findings do not apply to conditions applicable to industrial waste gas purification for which no conclusions can be drawn from the results given. It is apparent from the prior publications that it is extremely difficult to obtain complete oxidation of sulfur dioxide to give sulfate sulfur at the relatively high temperatures applicable in dry waste gas purification.
The Journal of the Institute of Fuel, September 1956, pages 372-380, for example, describes that sulfur trioxide reacts with ammonia selectively to form sulfate at elevated temperatures (150.degree. C.-650.degree. C.) and in the presence of water vapor. Under these conditions, no reaction between sulfur dioxide and ammonia takes place whereby the sulfur dioxide could be separated. It is considered necessary that oxidation of sulfur dioxide be carried out beforehand. In U.S. Pat. No. 2,912,304, there is described an oxidation process wherein solid ammonium sulfate is prepared from gases containing sulfur dioxide by adjusting the stoichiometric gas mixture of sulfur dioxide, ammonia, water and NO.sub.X at temperatures above 200.degree. C. in a fluidized catalyst bed wherein vanadium pentoxide is used as a catalyst. The disadvantage of this process is that the particles of catalyst are inactivated because the particles become caked with ammonium sulfate. Expensive regeneration of the catalyst must be carried out. The problem of caking in pipelines and apparatus also occurs with other processes described above which require expensive purification systems.
In West German Patent No. 2,431,130, the object of the disclosure is to produce dry ammonium sulfate from the sulfur dioxide and sulfur trioxide content in the waste gas. The dry salt is obtained by injecting a saturated solution of ammonium sulfate into the flow of hot waste gas and then passing the mixture through an electrostatic precipitator at a downstream location. The amount of heat withdrawn from the waste gas is relatively low, so that in this process, the temperature of the gas remains far above the dew point of the gas. The salt solution to be vaporized is produced in this process by bringing the gas into contact with a circulating solution in a downstream wet scrubber. The circulating solution contains a cation, e.g., ammonia, which absorbs sulfur dioxide. The reaction takes place relatively slowly at the interface of the liquid film and, therefore, internal members in the column, such as Raschig rings, are required to promote the mass and heat transfer. Cooling of the gas in this process occurs by yielding sensible heat from the gas for concentration of the circulating solution. The considerable loss of heat must be made up by the use of expensive completely desalinated water, as is apparent from the specification. Another disadvantage of supplying fresh water into the top of the column is that the gas flow from the column is at dew point temperature while charged with the additionally-evaporated water. A further disadvantage is that the circulating solution consists of a mixture of sulfate and sulfite which, before evaporation, must be oxidized with air or with ozone-enriched air.
The ammonium sulfate obtained from this process can be used without processing in expensive purification plants only when the fly ash content of the treated waste gas is very low and if the waste gas contains no harmful constitutents to the environment. This known process is unusable for waste gases containing other constituents harmful to the environment, e.g., hydrogen chloride, hydrogen fluoride and the like.
West German Patent No. 2,518,386 discloses an improvement to the above-described process, wherein ammonia is fed to the flue gas before or immediately after mixing with the concentrated salt solution. This is described to have the advantage that the dew point temperature of the sulfuric acid is so lowered as to be far below the gas temperature. This does not eliminate the above disadvantages.
A sulfur dioxide absorbing cation is again formed in the bottom part of the wet scrubber by the circulating solution. The water losses must be made up by a completely desalinated supply of fresh water. In addition, ammonium sulfate production is no longer economic because ammonium sulfate can be used profitably only with difficulty. Moreover, ammonium nitrate is preferred to ammonium sulfate for fertilizing purposes. The ammonia sulfate salt cannot be discharged into rivers because the quantity of salt and nutrient in the water would be increased with polluting effect. No comprehensive solution has heretofore been disclosed in the art to provide for the separation and utilization of byproducts from waste gas processing which is both compatible with the environment and yet economic.
This applies particularly to the case in which harmful constituents, such as hydrogen chloride, hydrogen fluoride and nitrogen oxides are to be removed from waste gases at the same time as sulfur dioxide and sulfur trioxide are removed from the gases. This is the object of the invention.